Data transducer position control system for rotating disk data storage equipment

ABSTRACT

An improved apparatus and method is disclosed for controlling the position of one moveable member relative to another. In the disclosed preferred embodiment of a rotating disk memory system, the moving member is a head carriage structure and the other is a frame carrying a rotating data storage disk. From externally supplied position selectron information, and from internally determined polyphase position information, open loop position changing movements are determined and carried out in accordance with monitored incremental polyphase position information generated during the movement. When a destination location has been reached one or more servo systems close loop in order to maintain the moving member precisely at the newly commanded location. A rotor for moving the member is an aspect of the present invention.

BACKGROUND OF THE INVENTION

This invention relates to position control systems and methods fortranslating one member relative to another, and more particularly, thisinvention relates to methods and apparatus for moving a memory datadevice member such as a transducer relative to another member such as adesired concentric track of a rotating disk, and keeping the movedmember in desired alignment with the other member, e.g. the transducerin registration with the track.

In electromechanical devices such as the rigid rotating memory devicesdisclosed by the prior art, two basic approaches have been taken inorder to position data transducers radially relative to the rotatingmagnetic disk surface. A first, high cost approach was to utilize adedicated servo system with a servo head and a replicated servo surfaceon the disk to provide a high performance track-following transducerpositioning scheme. On the other hand a more recent, low cost approachwas to utilize completely open loop stepping motor positioners whichoperated to place the transducers at arbitrarily defined tracks withoutany actual position information being fed back from the disk to the headpositioner. The main drawback of the prior art low cost open loopapproach was the requirement that tracks be spaced far enough to takeinto account all of the variations of the system, including mechanicaltolerances in the stepping motor actuator, thermal expansion of thedisk, and disk run-out. The result was a disk drive product which, whileeffective as a low cost unit, lacked the data storage capacity of themore expensive units with the result that the cost of storage per bitstored of the low cost drives approached the same cost as the earlier,more expensive storage units.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a novel apparatus forpositioning one member such as a data transducer relative to anothermember, such as a rotating data storage medium.

Another object of the present invention is provide a low cost closedloop servo control system which combines the best features of the priorart to improve the accuracy of maintaining a transducer in alignmentwith a data track during read and/or write operations of a disk havinghigh data track densities.

A further object of the present invention is to provide an improved yetmore reliable open loop position seeking system which overrides a closedloop position maintaining servo control when seeking from one positionto another position.

Yet another object of the present invention is to provide a lightweight,substantially linear, pure torque driven transducer carriage for rapidlymoving the transducer radially across the data tracks during trackseeking operations and to maintain the transducer on track during dataread and write operations.

A still further object of the present invention is to provide a uniquesingle servo sector on the rotating disk which contains track centerlinedata capable of being read by the transducer and converted to an offsetvalue to provide a fine adjustment to the head carriage in order tomaintain the transducer at the centerline of the track during read andwrite operations.

Yet one more object of the present invention is to provide an improved,yet simplified high speed and pure torque producing position translatorfor positioning one member relative radially to another member.

One more object of the present invention is to combine readily availableand inexpensive electrical and mechanical components in a unique way toprovide an improved position control system which occupies a smallphysical space, which requires only a modest power supply, which isinexpensive to manufacture and which operates reliably over a longuseful life.

These and other objects of the present invention are obtained inelectromechanical equipment such as data storage systems which include aframe and a member such as a head support structure rotatably mounted tothe frame and moveable relative to said frame among preselectedavailable ones of a multiplicity of selectable positions such asconcentric data tracks of a rotating data storage disk.

The moveable member includes a bidirectionally moveableelectromechanical mover supported by said frame,

A bidirectional mover driver is connected to the mover for moving themember to maintain it at a selected one of the positions during amaintained position mode of operation and to transport the member from adeparture position to a destination position during a new positionseeking mode of operation.

A position transducer provides a polyphase signal, such as quadrature,which is generated in response to actual sensed present position of themember relative to the frame.

A position controller is connected to the position transducer, saidmover driver, and to an external source of new position selectioninformation. The controller records the present position of the memberrelative to the frame; it calculates a new position seeking command inresponse to known present position and the new position selectioninformation; and, it commands the member to move from the known presentposition to a requested destination position during a new positionseeking mode of operation, by commanding at the mover a first spatialincrement of maximum forward direction acceleration followed by asimilar spatial increment of maximum reverse direction acceleration andthen by commanding adaptively a slewing rate dependent upon incrementalpolyphase position information of the member provided by the positiontransducer until the destination position is reached.

A position-dependent closed loop servo is connected to the positiontransducer and to the mover driver for operatively controlling saiddriver to keep the member positioned within a selected one of thepositions during the maintained position modes of operation, the loopbeing opened during accelerative portions of new position seeking modesof operation.

Another aspect of the present invention, particularly applicable toelectromechanical systems such as rotating disk storage technologyprovides a fine position closed loop servo. It is connected to thedriver and operates from prerecorded information in a single, datamasked servo sector on a data surface of the rotating disk. This data isread by a head supported by the moveable member. A sample and holdcircuit is connected to the head during its passes over the sector andholds the control data read therefrom. A correction signal generator,connected to the sample and hold circuit and to the driver generates andsupplies an offset value which when applied to the driver promotes andmaintains track centerline alignment of the head during read and/orwrite operations of the disk storage system. The fine position loop isoverriden and ignored during accelerative portions of new positionseeking modes of operation.

A rotor provides another aspect of the present invention, and itincludes an even number of coil segments, which may be woundsequentially on a moving bobbin from a single strand of wire. The coilsegments are arranged adjacently in a thin disk and are connected toprovide bidirectional, symmetrical torque. Potting compound encapsulatesthe coil structure and provides a very high resonant frequency and acapability to absorb vibration energy from the rotatable member.

The method of the present invention practiced in the environment ofelectromechanical equipment such as data storage systems which includemoving a member such as head support structure relative to a frame amongpreselected available ones of a multiplicity of selectable positionssuch as concentric data tracks of a rotating data storage disk,including the steps of:

moving the member electromechanically relative to the frame in order tomaintain it at a selected one of the positions during a maintainedposition mode of operation and in order to transport the member from adeparture position to a destination position during a new positionseeking mode of operation,

generating a polyphase signal, such as quadrature, in response to sensedactual present position of the member relative to the frame,

receiving and storing new position selection information from anexternal position control source,

recording the present position of the member relative to the frame,

calculating a new position seeking command in response to known presentposition and the new position selection information,

commanding the member to move from the known present position to arequested destination position during a new position seeking mode ofoperation by

commanding a first spatial increment of maximum forward directionacceleration, then

commanding a similar spatial increment of maximum reverse directionacceleration, then

commanding adaptively a position crossing slewing rate dependent uponincremental polyphase information of the member provided by the step ofgenerating the polyphase signal until the destination position isreached, and then

stopping and holding said member at said destination position until thenext position changing movement is commanded, and

serving upon said generated polyphase signal for operatively maintainingthe member positioned within a selected one of the positions during themaintained position mode of operation, and opening up the servo loopduring accelerative portions of new position seeking modes of operation.

The invention, includes as other aspects thereof in a data storage diskdevice the further step of:

providing a single, data masked servo sector on a data surface of thedisk,

prerecording track centerline servo control data in the servo sector,

reading the servo data with a head passing adjacent to the disk surfacecontaining the control sector,

sampling and holding the read servo data,

generating an offset correction signal,

applying the offset signal to move the member so as to maintain andpromote centerline alignment of a data transducer head carried on themember with each selected concentric data track during read and/or writeoperations, to provide a fine position servo control loop, and

overriding the loop during accelerative portions of the track seekingoperations.

Other objects, advantages and features of the invention will be apparentto those skilled in the art from a consideration of the followingdetailed description of a preferred embodiment, presented in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is an overall system block diagram illustrating the principles ofthe present invention.

FIG. 2 is an enlarged and diagrammatic plan view of a wedge shapedportion of the rigid disk, illustrating the single servo pattern forinner and outer tracks, with the middle tracks broken away to savedrawing space.

FIG. 3, consisting of A-I, is a series of timing and waveform diagramsrelated to the operation of the system in response to sensing the trackservo sector information depicted in FIG. 2.

FIG. 4 is a block and schematic diagram of some of the circuitry of thesystem depicted in FIG. 1.

FIG. 5 is an enlarged sectional view in side elevation of the coarsehead position sensor assembly of the system depicted in FIG. 1.

FIG. 6, consisting of A-D, is a series of graphic representations of theoperational states of the sensor assembly depicted in FIG. 5.

FIG. 7, consisting of A-L, is a waveform diagram of the control signalsgenerated by the FIG. 1 system in response to the FIG. .[.4.]. .Iadd.5.Iaddend.sensor.

FIG. 8 is a waveform diagram depicting operation of the system of FIG. 1during a track seeking operation of approximately 120 tracks in radialdistance.

FIG. 9 is a somewhat diagrammatic view in side elevation and verticalsection of a disk drive and head carriage assembly in accordance withthe principles of the present invention.

FIG. 10 is a top plan view of the hexagonal wire-wound rotor of the headcarriage assembly depicted in FIG. 9.

FIG. 11 is a schematic wiring diagram of the wire-wound rotor of thehead carriage assembly depicted in FIG. 9.

FIG. 12 is a view in side elevation and section of a portion of thewirewound rotor depicted in FIG. 10 taken along the line 12--12.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, the hard disk memory system 10 illustrated inoverview therein is one in which a drive spindle 12 and one to fourapproximately eight inch diameter magnetic disks (two disks 14 and 16are depicted) are caused to rotate about the common axis of the spindleat a .Iadd.a constant velocity, e.g. 50 Hz .Iadd.(3000 RPM), .Iaddend.bya suitable disk drive motor 18 with pulleys and a drive belt 20 inconventional fashion. The system 10 may include as many as four or moremagnetic disks, and disk diameters such as fourteen or five and onequarter inches are useable, although eight inches is presentlypreferred. The disks 14 and 16 may be formed from thin aluminum sheethaving an oriented ferric oxide or other suitable magnetic coating onthe major surfaces thereof. While magnetic surface disks are described,the present invention may be effectively utilized with other kinds ofdata storage devices, including laser etched disks or optical storagedevices. .Iadd.Each major surface of each disk 14, 16 carries asmultiplicity of radially displaced concentric data tracks 00 through n,depicted in FIG. 2. 512 data tracks are presently preferred for eachdata surface in the system 10. A landing zone L is located inside theinnermost data track n. .Iaddend.An index marker 22 provided on thespindle (FIG. 9) is used to provide a tachometer or index clock signalwhich is used to control servo operations in a manner to be describedshortly and which also serves as a check to assure that the disks arerotating at the desired 50 Hz angular velocity.

A head carriage assembly 24 includes a pure-torque-generating rotor 26to which head support beams 28 are mounted for radial movement relativeto the disks 14,16. The rotor 26 is described in greater detailhereafter in connection with FIGS. 9-12. Read or write transducers(heads) 30 are secured at the periphery of the support beams 28, andthese heads may be of the type which ride upon an air bearing effect inaccordance with what has come to be known in the art as Winchestertechnology.

A coarse head position electro-optical transducer includes a controlledcurrent light emitting diode source 32, a scale 34 having a series ofequally closely spaced microscopic radial lines, and an integratedcircuit photo sensitive reticle-masked array 36, which in combinationproduce the light and dark polyphase (e.g. quadrature) patterns depictedin FIG. 6 and are used to generate the .Iadd.analog .Iaddend.sawtoothservo waveform .Iadd.G .Iaddend.depicted in FIG. 7.

There are five outputs from the photo sensitive array 36. Four of theoutputs are quadrature track position signals which are processed by thedifferential amplifiers 38 and 40. The fifth is a signal which indicateslocation of the head 30 at track zero (i.e. the radially outermostuseable data track) and it is amplified and shaped by an amplifier 42.The quadrature signals, .Iadd.e.g., waveform A of FIG. 7, .Iaddend.fromthe differential amplifier 38 and 40 are supplied to a positionlinearity switch detector circuit 44 which provides the waveformsdepicted in FIG. 7 B and C, and also to a position signal selectorcircuit 46, the operation of which is controlled by the switch detectorcircuit 44.

As depicted diagrammatically in FIG. 1, structurally in FIG. 5,optically in FIG. 6 and electrically in FIG. 7, the scale 34 rotateswith the head structure 28 relative to the frame. With this rotation thescale microlines pass between and interrupt the light beams passing fromthe source 32 to the detector array 36. With the geometry of thedetector array 36 being in accordance with the FIG. 6 sketches, fourdata tracks may be defined by each microline and space.

The geometry of the light sensitive photodiode array 36 is depicted inFIG. 6. Actually there are four pairs of detector windows, which areradially offset by a distance of a half of a microline. Each window pairsees four phases of each line: first half line, full line (full dark),last half line, and no line (full light). The window pairs are furtherpaired together diagonally. For example the top left window pair and thebottom right window pair provide the two differential inputs to theamplifier 38, and the bottom left and top right pairs are the inputs tothe amplifier 40. In FIG. 6A equal and oppositely phased light and darkareas in the upper left and lower right detector pairs provide a nulloutput defining one track. In FIG. 6B the scale has moved to the nextphase so that full lines are now seen in the top left windows. In thissituation the bottom left and top right are equal and opposite, and theamplifier 42 is at null point defining the next data track. In FIG. 6Cthe scale has moved yet another half line width and the third phasepattern presented is the same as FIG. 6A except for phase reversal. InFIG. 6D the scale has moved still another half line width to the fourthphase pattern, similar to the FIG. 6B pattern except for phase reversal.

The output of the position linearity switch detector 44 is depicted asthe FIG. 7-B and C waveforms. The output from the position signalselector 46 is an analog value which is supplied to a summing network48, and then on through a loop compensation (damping) network 50, abidirectional moving coil motion driving amplifier 52 and ultimately tothe armature of the rotor 26 via a bidirectional rotation driving line54. The analog value produces a correction torque to keep the heads 30within the boundaries of each data track defined by the scale 34 andphotodetector 36. The light emitting diode light source 34 is powered bya driver circuit 56 which includes an automatic light level (AGC)control developed from a sixth photodetector in the detector array 36.

The data read by one of the transducers 30 is selected, preamplified andlow pass filtered by the circuits denoted by the block 58 of FIG. 1.Thereafter, the reproduced MFM formatted data is recovered by a datarecovery circuit 60 and sent to the computer or other appliance to whichthe system 10 is connected for random access data storage and retrieval..[.One.]. .Iadd.A fine position closed loop servo system is employed bymemory system 10 to assist in maintaining alignment between heads 30 andthe centerlines of the data tracks on the surfaces of the disks 14 and16 during memory system read and write operations. To this end, one.Iaddend.disk surface 14 may be provided with a narrow, 200 byte wide.Iadd.data masked .Iaddend.sector 62, which is depicted diagrammaticallyin FIG. 2 and electrically by the FIG. 3 waveforms. Each data track,from track zero zero to track n (e.g. track 511) is provided with thetwo factory prerecorded frequency bursts, a first occurring burst B1 onthe outside half of e.g. odd tracks (and inside half of e.g. eventracks), and a second occuring burst B2 on the inside half of odd track(outside on even tracks). .Iadd.Bursts B1 and B2 are prerecorded at aposition on the disk surface which is determined by the occurrence ofthe index clock signal generated from spindle index marker 22. Inparticular, index marker 22 is detected by a detector 78 and, as seen inFIG. 1, instead of being sent directly to the disk drive interface 88 issent to a digital system controller microprocessor 76 to mark thelocation in time of the servo sector on the disk. In the embodiment ofFIG. 2, bursts B1 and B2 immediately follow in time the occurrence ofthe index clock signal generated from spindle index marker 22 when disk14 is rotating at proper velocity in the direction of arrow 63. When thesingle servo sector so located in relation to the index clock signal,the data masked servo sector is established. The read/write data canthen be stored and retrieved in each cylindrical data track followingthe single servo sector on the disk surface in a data format which issubstantially unrestricted by the single servo sector. .Iaddend.Thesector bursts .Iadd.B1 and B2 .Iaddend.are read .Iadd.once .Iaddend.each.Iadd.disk .Iaddend.revolution by the head 30 and are used to provide afine head position servo loop control signal to the rotor 26 in the formof an offset voltage to urge the head 30 into alignment with thecenterline of the track. Each burst is read in turn and integrated bythe peak detector 66 to provide the amplitudes thereof. These amplitudesare then sampled and held by circuits 68 and 70. The held values arecompared by an analog data selector 72, and a signal analog differenceis converted to an eight bit digital value by an analog to digitalconverter 74. This digital value is processed in .[.a.]. digital systemcontroller microprocessor 76, such as the Intel type 8048 which containsa 1k byte factory preprogrammed read only program memory and a 128 byterandom access scratchpad memory.

A detector 78 detects the index mark on the drive spindle 12 with eachrevolution. This index clock signal is passed through an amplifier 80and sent to the microprocessor 76 to provide a digital tachometer todetermine whether the disks are rotating at correct speed and to markthe location in time of the servo sector on the disk. The index clock isalso processed by a servo sample timer 82 which is used to enable andswitch between the sample and hold circuits 68 and 70.

The waveforms of FIG. 3 illustrate the operation of the circuit elements30, 58, 64, 66, 68, 70, 72, 74, 76, 78, 80, and 82 which provide thefine position servo. Waveform A depicts the 50 Hz index pulse Igenerated by the index detector 78. Waveforms B and C depict the firstoccurring Burst 1 and second occurring Burst 2. Waveform D depicts theservo sector data window which immediately follows each index pulse,.Iadd.and which defines in time the data masked servo sector..Iaddend.Waveform E shows the control signal from the servo sample timer82 as it is applied to the sample and hold circuits 68 and 70; itdivides the sector into two 100 byte halves. Waveform F shows theamplitude of a first burst A stored in the first sample and hold circuit68. Waveform G shows the amplitude of the second burst B stored in thesecond sample and hold circuit 70. Waveform H depicts equivalence ofsensed amplitudes which obtains when the head is properly aligned withinone data track. In this situation no offset value is required and onewill be supplied by the microprocessor 76 to the rotor 26. Waveform Idepicts a much larger first burst than second burst which indicates thehead is not on center but is close to an edge of the track. .Iadd.Inthis manner, microprocessor 76 is able to take into account positionerrors such as those caused by thermal expansion of the disks 14 and 16as internal ambient temperatures rise. .Iaddend.

It will be appreciated by inspection of FIG. 2 that servo bursts oninnermost tracks n-3, n-2, n-1 and n are much smaller in amplitude thanbursts on outermost tracks 00, 01, 02, 03 and 04. Consequently, in orderto calculate a valid offset signal for fine position servo purposes, itis necessary to calculate the percentage of difference between burstamplitudes with the microprocessor 76. This calculation automaticallyprovides an automatic gain control (AGC) signal for each track, a signalwhich may be applied to the data recovery circuit 60 or to othercircuits for providing gain equalization for recovered data.

A disk drive interface circuit 88 receives control information from thehost computer, etc. and supplies that information, including track seekdata to the microprocessor 76. Data surface/head select information issent directly to the head select circuit 58. The microprocessor 76always knows where the head is presently located because of a two bitquadrature signal line from the switch detector 44. The microprocessor76 determines how far and in what direction to move the head (seek) andthen it calculates a set of numbers which are put out in a sequencedependent upon actual head position during the seek operation.

Some of the major elements of the coarse position closed loop servo andthe digital override circuitry are depicted in FIG. 4. Therein, thelatch 84 is depicted as a standard TTL type 74LS374 latch which isclocked by an input from the WRITE line of the microprocessor 76. Thedigital to analog converter (DAC) 86 is implemented as a Motorola typeMC1408L8, and it receives the latch 8 bit digital numbers of offsetvalues from the latch 84 and converts them to control currents. Avoltage reference circuit 102 is utilized to reference the DAC to systemvoltages. An operational amplifier current to voltage converter 106buffers and scales the resultant analog control voltage from the DAC 86.The summing circuit 48 in the implementation depicted is FIG. 4.[.occurs at.]..Iadd., which is electrically connected to .Iaddend.theinput of the driving amplifier 52 .[.which.]..Iadd.,.Iaddend.is suppliedwith the analog offset voltages from the DAC 86 and the coarse servoloop voltages from the position signal selector 46.

The circuitry of the bidirectional driving amplifier 52 emulates theoperation of a differential output amplifier. To do this, .[.thecircuitry includes.]. two amplifiers 108 and 110 .Iadd.are.Iaddend.wired as shown in FIG. 4. The analog controls to the drivingamplifier 52 occur at the input of the op amp 108, and the digital seekoverride controls are applied directly to the inputs of the twoDarlington pair power drivers 112 and 114 which drive the two windingsA-B and B-C of the rotor. (See discussion of FIG. 11, below). The op amp110 functions to make the characteristics of the driver 114 opposite andcomplementary to the input of the driver 112.

An operational amplifier 116 .Iadd.connected to amplifier 108.Iaddend.receives two complementary coarse servo loop control signals Pand bar P from the position signal selector 46. These values are equaland opposite and of minimum amplitude when the head carriage 28 is ingeneral alignment within any given defined track. The P and bar P valuesare derived by the position signal selector 46 from the .[.digital.].quadrature waveforms .[.K and L.]. depicted in FIG. 7 .Iadd.A, using theFIG. 7 digital waveforms. The operational amplifier in turn outputs ananalog value in the form of the sawtooth servo waveform depicted in FIG.7 G and supplies this analog value to the negative input of amplifier108. The positive input of amplifier 108, meanwhile, receives theaforementioned analog offset voltages from DAC 86 via current to voltageconverter 106. .Iaddend.

An accelerate and decelerate combinatorial logic array 118 accepts a twobit word derived from the high order data bit clocked from the latch 84,and a binary control line from the microprocessor 75 designated the SEEKenable line. The logic 118 provides digital outputs which are bufferedand inverted and are then applied to the input of the power amplifier112 via a line 120 and to the inverted input of then amplifier 114 via aline 122 (and the inverting op amp 110.) The power amplifiers 112 and114 may be implemented as type TIP 140 power .[.Darlingron.]..Iadd.Darlington .Iaddend.pairs which are thermally sumped into a casealuminum frame 100 (FIG. 9) of the system 10. The operational amplifiers106, 108, 110, and 116 may be type 741, or equivalent.

A power supply switch 124 switches on the power supply to the amplifier52 only when it concurrently receives three enabling signals: a signalindicating presence of the required supply voltage, a READY controlsignal from the microprocessor 76, which denotes that the disks 14, 16are spinning at operating velocity, and an Drive Inhibit Line signal. Inthe event of a loss of operating velocity, power supply potential, or incase of a system reset, power is removed from the driver amplifier 52,and the head carriage assembly 28 automatically returns to the innerlanding zone in response to a bias spring 194 (FIG. 9) which operates indefault of the rotor 26.

The individual circuit components depicted in FIG. 4 are connected asshown and will not be discussed further, since the values andconnections are readily derived by those skilled in the art.

Referring to FIG. 1, an eight bit data word output from themicroprocessor 76 is supplied to a data latch 84 which is clocked by theWRITE enable line of the microprocessor 76. Each eight bit number heldin the latch 84 is converted to an analog value by the digital to analogconverter 86. The high order latched bit, along with another controlline direct from the microprocessor 76, is applied directly to thedriving amplifier 52 so as to override the coarse position servo system(elements 24, 38, 40, 44, 46, 48, 50) during the maximum accelerativeand decelerative phases of a track seek operation. In the final phase oftrack seek, a predetermined velocity slewing rate is achieved byrecurrent number series which are converted to analog values by theconverter 86 and applied to the summing circuit 48. During the initialmaximum accelerative and decelerative step function portion, the coarseservo loop is completely overriden. At about eight tracks fromdestination track, the maximum deceleration command is removed justbefore the carriage assembly 24 ceases to move. Thereafter, the coarseservo is commanded to slew across each track by a progressive analogstaircse signal which is reset to zero with each detected trackcrossing. In this way, the coarse servo, under the command of themicroprocessor 76, operates solely upon the position information derivedfrom the transducer 24 and irrespective of actual instantaneous velocityof the head carriage assembly 24.

If a track crossing occurs sooner than expected, then the staircasecommand resets to zero before a maximum value is reached. If a trackcrossing is delayed, then the staircase reaches and holds its maximumvalue until the transition. These conditions are illustrated in FIG. 8A.

Finally, a short deceleration step function may be applied to stop thecarriage assembly 24 at the destination track should its velocity notthen have reached zero. .Iadd.Reading of the fine servo and updating ofthe analog offsets can then be performed by microprocessor 76..Iaddend.FIG. 8A depicts the digital and analog waveform utilized tocommand a track seek across e.g. 120 tracks. FIG. 8B portrays thevelocity of the head carriage 24 relative to radial distance across the120 tracks.

When the destination track has been reached, the system 10 enters atrack extension control mode. One implementation of this mode is toselect the appropriate monophase of the quadrature signal and servo overits full cycle (i.e. a distance of plus or minus two tracks). In thisway the radial range of servo loop control covers a full four tracks,and only in the event that the head structure 28 is significantly jarredor otherwise is externally caused to move beyond plus or minus twotracks, will the coarse servo lose control. Upon a loss of servo controlthe system 10 enters a reset mode which resets the system and thenreturns the head 30 to the last selected track.

The microprocessor 76 has essentially five modes of operation or tasks:initialization, fine servo offset supervision, track seeking, emulationof other disk drive products, and self diagnostics. In theinitialization or start up mode when power is first applied, themicroprocessor 76 counts index pulses and compares them with an internalclock, to be sure that the disk spindle 12 is rotating at the propervelocity (50 Hz). The head 30 is initially located in a nonabrasivelanding zone L (FIG. 2). The microprocessor 76 initially commands a seekto the outermost track (track 00). When the head 30 reaches thisoutermost track, a special output is obtained from the transducer 24through the amplifier 42 (FIG. 7-J). The microprocessor 76 thencalibrates the fine servo by measuring, establishing and rememberingoffsets for the four outermost tracks 00, 01, 02, 03, and then for thefour innermost tracks n-3, n-2, n-1, n. If there is any difference ininitial calibration between the outermost and innermost tracks, themicroprocessor spreads this difference e.g. linearly over the totalnumber of tracks of the system. The microprocessor 76 then commands thehead 30 back to track 00. Initialization is them complete.

During read and write operations, the microprocessor 76 reads the fineservo continuously and updates the offset, to take into account positionerrors such as thermal expansion of the disks 14, 16 as internal ambienttemperatures rise.

As already explained, the microprocessor 76 receives seek commandsdigitally from the host computer via the disk drive interface 88. Themicroprocessor 76 maintains head position data in a register whichcounts the tracks from information derived from the coarse positiontransducer 24. The difference between the track at which the head ispresently located and the track sought, together with the sign value ofthe difference, which indicates the direction of head movement requiredto accomplish the seek, is used to calculate the series of commands fromthe microprocessor 76 to the rotor 26.

The emulation function of the microprocessor 76 enables the system 10 toemulate the characteristics of other disk drives. One such emulationwould be of the SA 1000 eight inch disk drive manufactured by ShugartAssociates, a XEROX Corporation subsidiary located in Sunnyvale, Calif.The SA 1000 product utilizes two adjacent read/write heads, and themicroprocessor 76 enables the system 10 to appear to a user as if twoheads are actually physically present in the system. This emulation isreadily provided by redefining the track counting structure in themicroprocessor 76 into two series of interleaved odd and even tracks,and then reading odd tracks as though it were with one of the heads, andreading even tracks as though it were with the other of the heads. Othercompetitive disk drive equipment may be emulated by special programmingof the microprocessor 76.

The self diagnosis function of the microprocessor 76 has short term andlong term aspects. During operation, the microprocessor 76 constantlymonitors disk rotating speed and head position. In the event of adiscrepancy in either parameter, the microprocessor takes the disk outof service and informs the host machinery of the detection of an error.Other errors and error messages are easily included, including thoseparticularly adapted to a data format or end use. Diagnostic routinesmay be contained in the read only memory of the microprocessor 76 orthey may be recorded on one or more of the tracks of the disk 14, andcalled by the microprocessor 76 as required.

Referring now to FIG. 5, the coarse head position servo transducer 24 isdepicted in enlarged structural side elevation and vertical crosssection. The transducer 24 is a U-shaped assembly comprising an uppermember 132 which supports the photodetector and reticle array 36, and alower member 134 which is keyed to hold the LED light source 32 invertical alignment with the photodetector 36. The scale 34 is a glassmember having equally spaced apart chrome microlines deposited thereon.It is precisely and securely attached to the head support structure 28e.g. at the rotor 26. The transducer 24 is mounted on a post 136embedded in the cast aluminum frame 100 which securely supports all ofthe disk drive machinery.

A feature of the present invention is that the transducer 24 isadjustable in two dimensions with but one point of attachment to theframe 100. A vertical height setting screw 138 and lock washer 140enables the members 132 and 134 to be adjusted up and down, so that thephotodetector can be adjusted to within five thousandths of inch of thescale 34 to achieve the required resolution. A spring 142 biases themembers 132 and 134 away from the frame 100.

Sideways alignment of the transducer 24 with respect to the scale 34 isachieved by rotating the members 132 and 134 about the axis of the post136. A locking screw mechanism 144 locks the members 132 and 134 thepost 136 at the desired sideways alignment.

The structural configuration of the system 10 is depicted in FIG. 9.Therein the drive hub 12 supports four disks, a top servo sectorcontaining disk 14 and three lower disks 16. The index detector 22 isprovided in a lower outer flange of the spindle 12, the sensor 78 issecured through the frame 100. The spindle 12 is mounted to a spindleshaft 152 by a screw 154 and a washer 156. Ball bearing assemblies 158,160 are placed in a cylindrical portion of the frame 100 and are held ina spaced apart configuration by a spring 162. A magnetic fluid seal 164is placed above the bearing 158 and seals the bearings by magneticcohesion of the sealing fluid. A bottom screw 166 secures a pulley, notillustrated, to the shaft, for the drive belt 20 from the motor 18.

The rotor 26 is depicted in FIGS. 9, 10, 11 and 12. In the FIG. 9vertical cross section, the rotor 26 includes a hub 172 to which thehead structure assembly 28 is mounted. A flat coil assembly 174 issecured by a bonding means such as adhesive to the base of the hub 172.Immediately below the coil assembly 174 is a ferroceramic permanentmagnet 176 which is fabricated as a unitary structure and thenmagnetized into a series of adjacent even-number opposed field magneticsegments in which the north and south poles alternate at the top andbottom of the magnet 176. The number of separate segments in the magnet176 corresponds to the number of coil windings in the coil assembly 174.As shown in FIGS. 10 and 11 there are e.g. six coil windings, as thereare e.g. six separate magnetic segments in the magnet 176. An annularflux return plate 178 of low carbon steel forms a base for the magnet176. In fabrication, a ceramic blank is glued to the base plate 178 andthen the resultant structure is permanently magnetized. A fixed shaft180 extends from a ribbed portion of the frame 100. The hub 172 isjournaled to the shaft 180 by ball bearing assemblies 182 and 184 whichare initially held in place during fabrication by an axial preloadspring 186 and spacer 188, with adhesive locking the bearings 182, 184to the hub 172.

A flux return top plate 192 is secured to the frame 100. A bias spring194 extends from a standoff on the top plate 192 to the head carriageassembly 28 and biases it to return to the inner landing zone of thedisks when power is removed from the rotor 26. Crash stops may be formedin the top plate 192 to limit range of head structure travel.

Other elements depicted in FIG. 9 include a printed circuit board 196which carries the circuitry immediately associated with the opticaltransducer 24 including the elements 38, 40, 42 and 56 depicted in FIG.1 and also the wiring connection for the heads 30. A main printedcircuit board 198 carrying the rest of the circuitry of FIG. 1, isplugged into the board 196 at a plug 200. A plastic case 202 mates withthe base 100 and provides an airtight seal throughout which is requiredfor the reliable operation of flying head Winchester drive technology.An air filter 203 fits into a recess of the frame, and fins 204extending from the flange at the base of the spindle 12 force air withinthe plastic enclosure 202 to pass through the filter 203. A breatherfilter 206 enables internal and external pressures to equalize.

To develop balanced torque or what we call "pure torque" which minimizesloading of bearings 182, 184, and enables very inexpensive bearings tobe utilized, we have developed a coil assembly 174 comprising sixseparate coil windings 212, 214, 216, 218, 220, 222, each of which isgenerally triangular as shown in the FIG. 10 plan view and all of whichmay be wound from a single continuous wire strand, as shownschematically in FIG. 11. Three coils 212, 214, 216 are series connectedtogether per the FIG.11 schematic (A to B), while the other three coils218, 220 222 are series connected together (B to C) in opposite phase tothe first three coils. It will be appreciated that the rotor 26 mustrotate in both directions. Use of two sets of opposed geometricallysymmetrical coils enables this movement through a simple switching ofthe power supply from one winding set to the other. Each coil segmentmay be wound upon a generally triangular bobbin, the three connectionsA, B, C, established and then leads connected and extending to theoutside. Each segment, in cross section (FIG. 12) is approximately nineturns across by 22 turns high, of 29 gauge copper wire. The coilassembly 174 is then placed in a forming mold, and an epoxy pottingcompound 224 is then placed and pressed into the mold to form theassembly 174 (FIG. 12). Adhesive is used to bind the coil assembly 174to the hub 172. By utilizing potting compound 224 to form the assembly174, a very high mechanical resonant frequency is achieved, and theassembly 174 dampens resonant vibration otherwise occuring in the headstructure 28, thereby increasing the mechanical bandwidth of the coarseservo loop.

The coil assembly 174 is configured so that adjacent coil windings willproduce equal and opposite forces resulting in a pure torque evenlyapplied about the axis of rotation of the rotor 26. If equal currentsare present in both sides of the coil assembly 174 (A to B and C to B)these forces cancel, and there is no resultant torque produced. Duringseeking, in the event of any position error or disturbance occurringwhile the system 10 is operating in the closed loop track following orslewing modes, the current in the winding becomes imbalanced, and thiscondition yields a restoring torque which moves the head structure untila position yielding equalibrium in current is again reached.

Counterweights, not shown, are added to the rotor hub 172 to place thecenter of mass of the head structure 28 in alignment with the axis ofrotation of the rotor 26. In this way, no unbalanced forces pass throughthe rotor bearings 182 and 184 as the rotor rotates about its axis.

It will be apparent to those skilled in the art that the presentinvention may be embodied physically in a wide variety of ways and withmany different elements and components. For example, the opticaltransducer 24 may be implemented magnetically. Alternatively, transducerinformation may be provided in the servo sector 62 and read with eachrotation of the disk 14. The rotor 26 may be replaced with an electricaldetent, microstep providing stepping motor, and the combination of sucha stepping motor with the time sampled fine servo loop will vastlyimprove track centerline following performance in a disk drive. Theservo loop systems of the present invention are advantageously, but notnecessarily, embodied in rotating magnetic disk storage devices. Otherservo applications, such as in spectrophotometers and other machinescharacterized by highly interactive mechanical elements are within thescope of the present invention.

Having thus described an embodiment of the invention, it will now beappreciated that the objects of the invention have been fully achieved,and it will be understood by those skilled in the art that many changesin construction and circuitry and widely differing embodiments andapplications of the invention will suggest themselves without departurefrom the spirit and scope of the invention. The disclosures and thedescription herein are purely illustrative and are not intended to be inany sense limiting.

We claim:
 1. In a data storage device including a frame, a data storagedisk journalled to the frame and rotatingly driven by motor means at aconstant velocity and characterized by a multiplicity of concentric datatracks on a major surface thereof, at least one data transducer headstructure in close proximity to said disk surface, the improvementcomprising:a bidirectionally rotatable electromechanical rotorjournalled to said frame adjacent to said disk, a head mountingstructure securing said head at one end thereof and secured to saidrotor at the other end thereof for moving said head across saidmultiplicity of concentric data tracks, .Iadd.bi.Iaddend.directionalrotor driver means connected to said rotor for rotating said structureto maintain it within a selected data track during read and/or writeoperations and to move said structure from a depature track to adestination track during track seeking operations, a head positiontransducer providing a polyphase position signal in response to sensedactual position of said head structure relative to said frame, headcontroller means connected to said head position transducer, said rotordriver and to an external source of track selection information, forrecording the present track position of said head structure, forcalculating a track seeking command in response to known head positionand said track selection information, and for commanding said head tomove from a known depature track to a requested destination track duringa track seeking operation .[.by commanding at said rotor a first headposition spatial increment of maximum forward direction accelerationfollowed by a similar head position spatial increment of maximum reversedirection acceleration and then by commanding adaptively a slewingrate.]. dependent upon said polyphase incremental head structureposition information from said head position transducer until saiddestination track is reached, a position dependent closed loop servoconnected to said head position transducer and to said rotor driver foroperatively controlling said driver to keep said head positioned withina selected one of said data tracks during read and/or write operations,said loop being open during track seeking operations until occurrence ofadaptively commanded slewing, a fine position closed loop servoconnected to said driver and further including a single, data maskedservo sector on a data surface of said disk containing track centerlineservo control data therein readible by a head of said structure adjacentto said surface, a sample and hold circuit connected to said head duringpasses over said sector for holding control data read therefrom, andcorrection signal generator means for generating an offset signal forapplication to said driver to promote and maintain track centerlinealignment of said head during read and write operations, said fineposition loop being opened during track seeking operation.
 2. In a datastorage device including a frame, a data storage disk journalled to theframe and rotatingly driven by motor means at a constant velocity andcharacterized by a multiplicity of concentric data tracks on the majorsurfaces thereof, at least one data transducer head in close proximityto a said disk surface, the improvement comprising:a bidirectionallymoveable electromechanical mover supported by said frame adjacent tosaid disk being moveable between and stabilized at a position related toeach said data track, a head mounting structure securing said datatransducer head at one end thereof and secured to said mover at theother end thereof for moving said head across said multiplicity ofconcentric data tracks, bidirectional mover driver means connected tosaid mover and capable of moving said structure to any selected one ofsaid concentric data tracks in accordance with externally supplied trackposition information and of maintaining said structure substantiallywithin said selected data track during read and/or write operations byreference to said frame, data track stabilization means including saidmover for stabilizing said structure within a range between theboundaries of each said data track, a time sampled fine position closedloop servo connected to said driver and including a single, data maskedservo sector on a data surface of said disk containing track centerlineservo control data therein readible by said head, a sample and holdcircuit connected to said head during passes over said sector forholding control data read therefrom, and correction signal generatormeans connected to said sample and hold circuit for generating an offsetsignal for application to said driver means to promote and maintaintrack centerline alignment of said head during read and writeoperations.
 3. In a data storage device including a frame, a disk drivemotor secured to the frame and connected to a source of electricity, amagnetic data storage disk rotatingly driven by said drive motor at aconstant velocity and characterized by a multiplicity of concentric datatracks on at least one major surface thereof, and at least one read andwrite head held in close proximity to said disk surface by air bearingeffect, the improvement comprising:a bidirectionally rotatableelectromechanical rotor journalled to said frame adjacent to said diskand having an axis of rotation parallel with the axis of said disk, abeam securing said head at one end thereof and secured to said rotor ata counterbalanced axis of rotation near the other end thereof forarcuately moving said head across said multiplicity of concentric datatracks, bidirectional rotor driver means connected to said rotor forrotating said beam to maintain said head within a selected data trackduring read and/or write operations and to move said head from adeparture track to a destination track during track seeking operations,a head position transducer mechanically linked to said beam andresponsive to absolute position of said head relative to said frame andproviding track position signals in quadrature, head controller meansconnected to said head position transducer, said rotor driver and to anexternal source of track selection information, for recording the trackposition of said head, for calculating a track seeking command inresponse to known head position and said track selection information,and for commanding said head to move from a known departure track to arequested destination track during a track seeking operation.[.bycommanding at said rotor a first head position spatial increment ofmaximum forward direction acceleration followed by a similar headposition spatial increment of maximum reverse direction acceleration andthen by commanding adaptively a slewing rate.]. dependent uponincremental quadrature crossing head position information from said headposition transducer until said destination track is reached, a positiondependent closed loop servo connected to said head position transducerand to said rotor driver for operatively controlling said driver to keepsaid head positioned within a selected one of said data tracks duringread and/or write operations, said loop being opened during trackseeking operations, a fine position closed loop servo connected to saiddriver and further including a single, data masked servo sector on saiddisk containing track centerline servo control data therein readible bysaid head, a sample and hold circuit connected to said head duringpasses over said sector for holding control data read therefrom, andcorrection signal generator means for generating an offset signal forapplication to said driver to promote and maintain track centerlinealignment of said head during read and write operations, said fineposition loop being opened during track seeking operations.
 4. The datastorage device set forth in claim 1 or 3 wherein said head controllermeans comprises a programmed microprocessor having a head position datainput connection from said head position transducer, a binary data latchfor receiving and holding said track seeking command, saidmicroprocessor clocking said latch in accordance with positioninformation derived from said transducer and said external source oftrack selection information, a data connection from said latch directlyto said driver means for enabling said controller means to command saidacceleration increments in said track seeking operations, a digital toanalog converter connected to said latch for converting slewing commandsclocked into said latch into analog values and for applying said analogvalues to said position dependent closed loop servo to command saiddriver means to slew said head securing beam structure at saidposition-adaptive slewing rate.
 5. The data storage device set forth inclaim 2 or 3 wherein said fine position closed loop servo comprises apair of prerecorded track marking signals in said servo sector for eachsaid defined track, a first occuring marking signal being radiallyoffset in one direction from track centerline, and a second occurringmarking signal being radially offset in the other direction from trackcenterline, first sample and hold means for sampling and holding theamplitude of said first occurring marking signal with each revolution ofsaid disk, second sample and hold means for sampling and holding theamplitude off said second occurring marking signal with each revolutionof said disk, and differential comparator means connected to said firstand second sample and hold means for comparing held ampitudes and forgenerating a head position correction signal proportional to thedifference in held amplitudes, and for applying said correction signalto said rotor through said driver means.
 6. The data storage device setforth in claim 5 wherein said differential comparator generates anautomatic gain control signal for automatically controlling andregulating amplitudes of data recovered in read operations of said diskby operatively regulating data recovery circuits of said data storagedevice.
 7. The subject matter as set forth in claim 1 or 3 wherein saidposition transducer comprises:a scale having equally spaced angularapertures defined thereon, and secured to one of said frame andstructural elements moved by said rotor, a unitized, generally U-shapedstructure having two parallel arms spaced apart for a distance slightlygreater than the thickness of said scale and aligned and mounted to theother of said frame and said structural elements moved by said rotor sothat said scale is free to pass between said arms, said structureincluding a light source in one of said arms for passing light throughsaid scale and in the direction of the other said arm, an array ofphotosensitive detectors and a light-pattern-defining reticle placedbetween said detectors and said scale in the other of said arms, saidreticle defining windows for said detectors which are so spaced as toprovide said polyphase signal in quadrature as said scale moves betweensaid arms.
 8. The subject matter set forth in claim 7 wherein said scaleis secured to said structural elements moved by said rotor and saidU-shaped structure is secured to said frame at a single location ofattachment and is adjustable with respect to said frame in two spatialdimensions.
 9. The subject matter as set forth in claim 1 or 3 whereinsaid rotor comprises a bidirectional, pure torque generatingelectromechanical rotor rotably mounted to a frame and rotatingly movinga member relative to said frame over a locus defining a sector of acircle, which is less than 90 degrees, said rotor comprising:a fluxreturn base plate, a flux return top plate, a generally annularpermanent magnet secured to one of said base and top plates andcharacterized by an even number of plurality of adjacently opposed fieldmagnetic segments in which the north and south poles alternate at themajor surfaces thereof, a generally annular, rotatable coil assemblyplaced and closely spaced from between said permanent magnet and theother of said base and top plates, said coil assembly containing thesame even number plurality of coils as there are magnet segments in saidpermanent magnet, said coils being aligned adjacent to said magneticsegments at at least one position of rotation of said assembly, saidcoils being connected into two series of opposed windings equidistantlyand symmetrically located about said assembly, a moving member attachedto said assembly and rotatably passing through at least one of said baseplate and top plate, said member being rotated along said locus bypassage of current through one of said series, said torque from saidcoils being applied evenly to said moving member about its axis ofrotation.
 10. The electromechanical rotor set forth in claim 9 whereinsaid rotor has been rotationally aligned relative to said magnet tolinearize torque amplitude over the range of said locus of movement. 11.In a data storage device including a rotating rigid magnetic medium datastorage disk, a read and write head held in close proximity to thesurface of said disk by the air-bearing effect, and a head carriagemechanism for positioning said head at one of a multiplicity ofconcentric data tracks during data read and/or write operations insubstantially unrestricted data format within each track and for movingsaid head from track to track during track seeking operations of saiddevice, the improvement comprising:driver means drivingly connected tosaid head carriage mechanism for arcuately moving said head from onetrack to a selected other track located beyond a predetermined minimumradial distance during an opened loop servo track seek movementcharacterized by a first spatial increment of maximum forward directionacceleration of said carriage for about half the radial distance betweensaid tracks and by a second similar increment of maximum reversedirection acceleration until said mechanism reaches .[.reaches.]. thevicinity of said selected other track, and for closed loop generallyconstant velocity microstepping of said mechanism within said minimumradial distance in small radial increments from track to track untilsaid other track is reached, and for maintaining said head accuratelywithin a selected track during data read and write operations, a summingcircuit connected to drive said driver means, coarse servo control loopmeans including coarse position sensor means mechanically linked to saidcarriage mechanism for sensing radial position of said head relative tosaid tracks, and coarse position feedback control loop means connectedto said driver means through said summing circuit for controlling saiddriver means to keep said head positioned within each said selected oneof said defined tracks during read and write operations, index detectormeans electromechanically coupled to said disk for detecting eachrotation thereof and generating an index signal, fine position servocontrol loop means including a single data-masked servo sector region onthe surface of said disk containing servo information therein, sampleand hold means connected to said head for sampling and holding saidservo information read in said servo sector for each revolution of saiddisk, correction signal generator means connected to said sample andhold means for generating offset signals, and analog to digitalconverter means connected to said correction signal generator means, forconverting said .[.offet.]. .Iadd.offset .Iaddend.signals into digitalvalues, programmed digital microprocessor means connected to said analogto digital converter means, said index detector means, said coarse servocontrol loop means and to a source of an externally supplied digitaltrack selection command signal, and including clocked latch means, anddigital to analog converter means drivingly connected to said drivermeans through said summing network for calculating and supplying adigital head positioning signal .Iadd.directly .Iaddend.to said drivermeans in order to move said head from a predetermined departure track toa predetermined destination track, .Iadd.said head positioning signaloverriding said summing circuit during acceleration phases of trackseeking operations, for calculating and supplying a track microsteppinghead positioning signal to said driver means through said digital toanalog converter means and said summing network during track seekmicrostepping, .Iaddend.and for calculating and applying a fine positionoffset signal to said driver means through said digital to analogconverter means and said summing network to maintain said head incenterline alignment in said track during read and/or write operationsinteractively with said coarse servo control loop means.
 12. In a datastorage method including the steps of rotating a data storage disk at aconstant velocity, providing a multiplicity of concentric data tracks onthe major surfaces thereof, reading and/or writing data from and to saidtracks with at least one data transducer head structure in closeproximity to said disk surfaces, and moving said structure from adeparture track to a destination track during track seeking operations,the improved method comprising:moving said head structure across saidmultiplicity of concentric data tracks with a bidirectionally rotatableelectromechanical rotor journaled to said frame adjacent to said diskand secured to said rotor at the other end thereof, rotating saidstructure with bidirectional rotor driver means connected to said motorto maintain said head within a selected data track during read and/orwrite operations and to move said head from a departure track to adestination track during track seeking operations, providing a polyphasesignal responsive to position of said head structure relative to saidframe with a head position transducer, providing track selectioninformation to head controller means connected to said head positiontransducer, said rotor driver and to an external source of trackselection information, and then recording the track position of saidhead structure, calculating a track seeking command in response to knownhead position and said track selection information, commanding said headto move from a known departure track to a requested destination trackduring a track seeking operation by commanding at said rotor a firsthead position spatial increment of maximum forward directionacceleration followed by a similar head position spatial increment ofmaximum reverse direction acceleration and then by commanding adaptivelya slewing rate dependent upon incremental head structure positioninformation from said head position transducer until said destinationtrack is reached, and operatively controlling said driver to keep saidhead positioned with a selected one of said data tracks during readand/or write operations with a position dependent, coarse positionclosed loop servo connected to said head position transducer and to saidrotor driver and overriding said coarse position loop to keep it openduring track seeking operations, providing a fine position closed loopservo connected to said driver and further including:providing a single,data masked servo sector on a data surface of said disk, prerecordingtrack centerline servo control data in said servo sector, reading saiddata with a head of said structure adjacent to said surface, samplingand holding data read by said head during passes over said sector andgenerating an offset correction signal, applying said offset signal tosaid driver to promote and maintain track centerline alignment of saidhead during read and/or write operations, and overriding said fineposition servo loop during track seeking operations.
 13. The datastorage method set forth in claim 12 and further comprising the steps ofproviding a pair of prerecorded track marking signals in said servosector for each said defined track by providing a first occurringmarking signal radially offset in one direction from track centerline,and providing a second occurring marking signal radially offset in theother direction from track centerline, separately sampling and holdingthe amplitudes of said first and second occurring marking signals, andcomparing said held amplitudes and generating therefrom a head positioncorrection signal proportional to the difference in held amplitudes ofsaid marking signals, and applying said correction signal to correcthead position until it coincides with track centerline.
 14. The datastorage method set forth in claim 13 and further comprising theadditional step of utilizing said comparison of held amplitudes togenerate an automatic gain control signal for automatic control andregulation of amplitude of recovered data.
 15. The data storage methodas set forth in claim 12 wherein said step of operatively controllingsaid driver to keep said head positioned within a selected one of saiddata tracks during read and/or write operations with a positiondependent, coarse position closed loop servo includes the step ofproviding a track extension control mode by selecting an appropriatemonophase of said polyphase position responsive signal and servoing overits full cycle.
 16. The data storage method as set forth in claim 12further comprising an emulation step for emulating operation of a diskdrive having two adjacent read/write heads, said step comprisingelectrically assigning odd numbered tracks to correspond with tracksread by one of said emulated adjacent heads and electrically assigningeven numbered tracks to correspond with tracks read by the other of saidemulated adjacent heads.
 17. In a data storage device including a frame,a data storage disk journalled to the frame and rotatingly driven bymotor means at a constant velocity and characterized by a multiplicityof concentric data tracks on the major surfaces thereof, at least onedata transducer head structure in close proximity to said disk surfaces,the improvement comprising:a bidirectionally moveable electromechanicalmover mounted to said frame adjacent to said disk, a head mountingstructure securing said head at one end thereof and secured to saidmover at the other end thereof for moving said head across saidmultiplicity of concentric data tracks, bidirectional mover driver meansconnected to said mover for moving said structure to maintain it withina selected data track during read and/or write operations and to movesaid structure from a departure track to a destination track duringtrack seeking operations, a head position transducer providing apolyphase position signal in response to sensed actual position of saidhead structure relative to said frame, head controller means connectedto said head position transducer, said mover driver and to an externalsource of track selection information, for recording the present trackposition of said head structure, for calculating a track seeking commandin response to known head position and said track selection information,and for commanding said head to move from a known departure track to arequested destination track during a track seeking operation bycommanding at said mover a first head position spatial increment ofmaximum forward direction acceleration followed by a similar headposition spatial increment of maximum reverse direction acceleration andthen by commanding adaptively a slewing rate dependent upon saidpolyphase incremental head structure position information from said headposition transducer until said destination track is reached, a positiondependent closed loop servo connected to said head position transducerand to said mover driver for operatively controlling said driver to keepsaid head positioned within a selected one of said data tracks duringread and/or write operations, said loop being open during track seekingoperations until occurrence of adaptively commanded slewing, a timesampled fine position closed loop servo connected to said driver andfurther including a single, data masked servo sector on a data surfaceof said disk containing track centerline servo control data thereinreadible once per disk revolution by a head of said structure adjacentto said surface, a sample and hold circuit connected to said head duringpasses over said sector for holding sampled control data read therefrom,and correction signal generator means for generating an offset signalfor application to said driver through said position dependent closedloop servo to promote and maintain track centerline alignment of saidhead during read and write operations, said fine position loop beingopened during track seeking operations.
 18. In a data storage deviceincluding a frame, a data storage disk journalled to the frame androtatingly driven by motor means at a constant velocity andcharacterized by a multiplicity of concentric data tracks on the majorsurfaces thereof, at least one data transducer head structure in closeproximity to said disk surfaces, the improvement comprising:abidirectionally moveable electromechanical mover mounted to said frameadjacent to said disk, a head mounting structure securing said head atone end thereof and secured to said mover at the other end thereof formoving said head across said multiplicity of concentric data tracks,bidirectional mover driver means connected to said mover for moving saidstructure to maintain it within a selected data track during read and/orwrite operations and to move said structure from a departure track to adestination track during track seeking operations, data trackstabilization means including said mover for stabilizing said structurewithin a range between the boundaries of each said data track, headcontroller means connected to said mover driver means and to an externalsource of track selection information, for commanding said mover to movesaid head from a known departure track to a requested destination trackduring a track seeking operation until said destination track isreached, a time sampled fine position closed loop servo including saidhead controller means and further including a single, data masked servosector on a data surface of said disk containing track centerline servocontrol data therein readible once per revolution by a head of saidstructure adjacent to said surface, a sample and hold circuit connectedto said head during passes over said sector for holding sampled controldata read therefrom, and correction signal generator means forgenerating an offset signal for application to said driver means throughsaid position dependent closed loop servo to promote and maintain trackcenterline alignment of said head during read and write operations. 19.The data storage device set forth in claim 18 wherein said fine positionclosed loop servo comprises a pair of prerecorded track marking signalsin said servo sector for each said defined track, a first occurringmarking signal being radially offset in one direction from trackcenterline, and a second occurring marking signal being radially offsetin the other direction from track centerline, first sample and holdmeans for sampling and holding the amplitude of said first occurringmarking signal with each revolution of said disk, second sample and holdmeans for sampling and holding the amplitude of said second occurringmarking signal with each revolution of said disk, and differentialcomparator means connected to said first and second sample and holdmeans for comparing held amplitudes and for generating a head positioncorrection signal proportional to the difference in held amplitudes, andfor applying said correction signal to said mover through said drivermeans.
 20. The data storage device set forth in claim 19 wherein saiddifferential comparator generates an automatic gain control signal forautomatically controlling and regulating amplitudes of data recovered inread operations of said disk by operatively regulating data recoverycircuits of said data storage device. .[.21. In a data storage methodincluding the steps of rotating a plurality of commonly journalled rigiddata storage disks at a constant velocity, providing a multiplicity ofaligned concentric data track locations on data storage major surfacesof said disks, reading and/or writing data from and to said tracks withcommonly mounted data transducers held in close proximity to said disksurfaces by air-bearing effect, and moving said structure from adeparture track to a destination track during track seeking operations,the improved method comprising:providing a bidirectionally moveableelectromechanical mover for maintaining each said transducer with aselected data track location during read and/or write operations and formoving said transducer from a departure track location to a destinationtrack location during track seeking operations, providing a transducerposition encoder for generating a polyphase signal responsive toposition of said mover relative to said frame and for providing binarytrack boundary marking signals, providing a reference track signalcorresponding to a reference track location, providing at least oneservo sector on a said surface with prerecorded track centerline servocontrol data for each data track location in the form of a firstoccurring marker burst of constant frequency radially offset in onedirection from track location centerline and a second occurring markerburst of constant frequency radially offset in the opposite directionfrom track location centerline, providing track selection informationfrom an external source, providing controller means including programmeddigital microprocessor means for:determining track location of saidtransducers by calibrating a digital track counter with said referencetrack signal and by incrementing and decrementing said track counterwith said binary track boundary marking signals as said mover moves saidtransducers away from and toward said reference track, calculating atrack seeking command in response to present track location of saidtransducers and said track selection information, commanding said moverto move said transducers from a known departure track location to aselected track location during a track seek operation by commanding saidmover to accelerate for a first spatial increment of movement and todecelerate for a second spatial increment of movement and by commandingadaptively a slewing rate dependent upon said binary track boundarymarking signals from said transducer encoder until said destinationtrack is reached, operatively controlling said mover by a control servoto keep said transducers positioned in substantial alignment with tracklocation centerline of a selected one of said data track locationsduring data read and/or write operations, by repeatedly: separatelyreading and holding peak amplitude values of said first and secondoccurring marker bursts, comparing said held values to provide an offsetsignal, calculating a transducer position correction signal from saidoffset signal, and applying said correction signal to said mover tocorrect transducer position to track centerline alignment..]. .[.22. Thedata storage method set forth in claim 21 further comprising initialtrack centerline calibrating steps of: calculating and recording headposition correction signals for a plurality of tracks lying in anoutwardly lying band of tracks, calculating and recording head positioncorrection signals for a plurality of tracks lying in an inwardly lyingband of tracks, averaging said correction signals of said outwardtracks, and averaging said correction signals of said inward tracks, andspreading the difference between said two averages uniformly over thetracks lying between said two bands..]. .[.23. In a data storage deviceincluding a frame, a plurality of non-removeable rigid rotating datastorage disks commonly journalled to said frame, a plurality of datatransducers travelling in close proximity to each major data storage andretrieval surface of each said disk upon an air-bearing cushion, andelectrically powered transducer mover means for commonly moving saidtransducer among aligned concentric data track locations of saidsurfaces in response to externally supplied track selection information,an improved transducer mover control system comprising:reference tracksensor means for sensing a reference track location, transducer positionencoder means for generating a polyphase signal responsive to positionof said transducer mover means relative to said frame and for providingbinary track boundary marking signals, at least one servo sector on asaid surface having prerecorded track centerline servo control data foreach data track location in the form of a first occurring marker burstof predetermined frequency radially offset in one direction from trackcenterline and a second occurring marker burst of predeterminedfrequency radially offset in the opposite direction from trackcenterline, controller means comprising programmed digitalmicroprocessor means, memory means, analog to digital converter means,and digital to analog converter means, said controller means having datainputs connected to said reference track sensor means, said transducerposition encoder means, and a said transducer reading said servo sectorbursts, and having a control output connected to said mover means, saidcontroller means for:recording track location of said transducers andincluding a digital track counter initialized by a signal from saidreference track sensor means and which is incremented and decremented bysaid binary track boundary marking signals as said mover means movessaid transducers relatively away from and toward said reference track,calculating a desired track seek command from recorded present tracklocation and said externally supplied track selection information,commanding said mover means to move said transducers from said presentlyrecorded track location to said desired track location, including meansfor commanding said mover means to accelerate for a first spatialincrement of movement and to decelerate for a second spatial incrementof movement and for commanding said mover means to move at an adaptiveslewing rate dependent upon said binary track boundary marking signalsuntil said desired track location is reached by said transducers, closedloop servo means for controlling said mover means to keep saidtransducers positioned in substantial alignment with track centerline ofeach selected data track location during read and/or write operations,and further comprising:peak amplitude detection and holding means forreading and holding peak amplitude values of said first and secondoccurring marker bursts, comparator means for comparing said held peakamplitude values to generate an offset signal, correction signalcalculation means for calculating a transducer position correctionsignal from said offset signal, and correction signal application meansincluding said digital to analog converter means for applying saidcorrection signal to said mover means to correct transducer position toachieve track centerline alignment..]. .[. . The subject matter setforth in claim 23 wherein said transducer position encoder meanscomprises:a scale having equally spaced angular apertures definedthereon, and secured to one of said frame and said mover means, aunitized, generally U-shaped structure having two parallel arms spacedapart for a distance slightly greater than the thickness of said scaleand aligned and mounted to the other of said frame and said mover meansso that said scale is free to pass between said arms, said structureincluding:a light source in one of said arms for passing light throughsaid scale and in the direction of the other said arm, an array ofphotosensitive detectors and a light-pattern-defining reticle placedbetween said detectors and said scale in the other of said arms, saidreticle defining windows for said detectors which are so spaced as toprovide said polyphase signal in quadrature as said scale moves betweensaid arms..]. .[.25. The subject matter set forth in claim 24 whereinsaid scale is secured to said mover means and said U-shaped structure issecured to said frame at a single location of attachment and isadjustable with respect to said frame in two spatial dimensions..]..[.26. The subject matter set forth in claim 23 wherein said mover meanscomprises a bidirectional, pure torque generating electromechanicalrotor rotatably mounted to a frame and a transducer support structurerotatingly moved by said rotor over a locus relative to said frame whichdefines a sector of a circle less than 90 degrees..]. .[.27. The subjectmatter set forth in claim 26 wherein said rotor comprises: flux returnbase plate means, flux return top plate means, permanent magnet meansbeing secured to at least one of said base and top plate means andcharacterized by a plurality of adjacently opposed field magneticsegments in which the north and south poles alternate at the majorsurfaces thereof, rotatable coil assembly means placed and closelyspaced adjacent to said permanent magnet means, said coil assemblycontaining the same even number of coils as there are magnetic segmentsin said permanent magnet means, said coils being aligned adjacent tosaid magnetic segments at at least one position of rotation of saidassembly means, said coils being connected into two series of opposedwindings equidistantly and symmetrically located about said assembly,said transducer support structure being attached to said assembly meansand rotatably passing through at least one of said base plate and topplate means, said support structure being rotated along said locus bypassage of current through one of said series, said torque from saidcoils being applied evenly to said support structure about its axis ofrotation..]. .[.28. The electromechanical rotor set forth in claim 27wherein said rotor has been rotationally aligned relative to said magnetmeans to linearize torque amplitude over the range of said locus ofmovement..]. .Iadd.29. A data track following method for providing auseful data storage capability in substantially unrestricted data formatthroughout a cylindrical data track on a data storage surface of arotating data storage disk in a data storage device and for providingthermal compensation in said device, said surface of said data storagedisk carrying a multiplicity of said data tracks, said data storagedevice having at least one data transducer held adjacent to said surfaceand moveable radially relative to said surface across said tracks and asingle electromechanical data transducer actuator for moving saidtransducer to each selected data track, said method comprising the stepsof:generating track boundary stabilization information by measuring theposition of said actuator relative to said disk, stabilizing said datatransducer at a position within the boundaries of each said selecteddata track in response to said stabilization information, closing a fineposition servo loop connected to said actuator by:generating a spindleindex signal, forming a single, data masked servo sector on said datastorage surface at a time related to said spindle index signal, saiddata masked servo sector containing track centering information, causingsaid transducer to read said track centering information prerecorded insaid single data masked servo sector once per revolution of said disk inresponse to said spindle index signal, sampling and holding saidcentering information, generating a track centerline offset correctionsignal from said held centering information, and applying said offsetcorrection signal to said actuator to promote and maintain trackcenterline alignment of said transducer during track followingoperations without otherwise interrupting said data track beingfollowed..Iaddend. .Iadd.30. The data track following method set forthin claim 29 wherein said step of generating said spindle index signalcomprises the additional steps of sensing physical position of a markeron a spindle to which said disk is journalled, and generating a servosector data window immediately following each said spindle index signal,and wherein said step of momentarily switching said transducer comprisesthe steps of switching said transducer to read said centeringinformation only during said servo sector data window..Iaddend..Iadd.31. The data track following method set forth in claim 29 whereinsaid step of generating track boundary stablization informationcomprises generating a polyphase signal responsive to the position ofsaid actuator relative to said disk, and wherein the step of stabilizingsaid transducer comprises the steps of selecting a monophase of saidpolyphase signal as corresponding in position with said selected trackand closing a coarse position feedback servo control loop including saidactuator to servo upon said selected phase..Iaddend. .Iadd.32. In arotating disk data storage device including a frame, a data storage diskrotatingly journalled to said frame by a spindle driven by spindle motormeans, said disk including a multiplicity of concentric cylindricaltracks on a major surface thereof, at least one transducer held in closeproximity at a selected track of said surface, a head support structurecarrying said transducer and moveably mounted to said frame andincluding a single electromechanical mover means for moving saidtransducer from said selected track to any of the others of said tracksduring track seeking operations and for stabilizing said transducer atthe vicinity of said selected track, an improved track following servosystem operative during track following operations, said systemcomprising: a single data masked servo sector on said surface of saiddisk prerecorded with track centering servo information for said track,sampling circuit means connected to said data transducer for samplingand holding said track centering servo information as said transducerpasses over said single sector once each revolution of said disk,correction circuit means connected to said sampling circuit means forgenerating an error signal related in value to spatial offset of saidtransducer from centerline of said selected track, and centerlineadjustment means responsive to said correction circuit means andoperatively connected to said mover means to enable it to move saidtransducer into alignment with the centerline of said selected track inresponse to said error signal..Iaddend. .Iadd.33. In a data storagemethod including the steps of rotating a plurality of commonlyjournalled rigid data storage disks at a constant velocity, providing amultiplicity of aligned concentric data track locations on data storagemajor surfaces of said disks, reading an/or writing data from and tosaid tracks with commonly mounted data transducers held in closeproximity to said disk surfaces by air-bearing effect, and moving saidstructure from a departure track to a destination track during trackseeking operations, the improved method comprising: providing abidirectionally moveable electromechanical mover for maintaining eachsaid transducer within a selected data track location during read and/orwrite operations and for moving said transducer from a departure tracklocation to a destination track location during track seekingoperations, providing a transducer position encoder for generating apolyphase signal responsive to position of said mover relative to saidframe and for providing binary track boundary marking signals, providinga reference track signal corresponding to a reference track location,providing a single, data masked servo sector on a said surface withprerecorded track centerline servo control data for each data tracklocation in the form of a first occurring marker burst of constantfrequency radially offset in one direction from track locationcenterline and a second occurring marker burst of constant frequencyradially offset in the opposite direction from track locationcenterline, providing track selection information from an externalsource, providing controller means including programmed digitalmicroprocessor means for:determining track location of said transducersby calibrating a digital track counter with said reference track signaland by incrementing and decrementing said track counter with said binarytrack boundary marking signals as said mover moves said transducers awayfrom and toward said reference track, calculating a track seekingcommand in response to present track location of said transducers andsaid track selection information, commanding said mover to move saidtransducers from a known departure track location to a selected tracklocation during a track seek operation by commanding said mover toaccelerate for a first spatial increment of movement and to deceleratefor a second spatial increment of movement and by commanding adaptivelya slewing rate dependent upon said binary track boundary marking signalsfrom said transducer encoder until said destination track is reached,operatively controlling said mover by a control servo to keep saidtransducers positioned in substantial alignment with track locationcenterline of a selected one of said data track locations during dataread and/or write operations, by repeatedly: separately reading andholding peak amplitude values of said first and second occurring markerbursts, comparing said held values to provide an offset signal,calculating a transducer position correction signal from said offsetsignal, and applying said correction signal to said mover to correcttransducer position to track centerline alignment..Iaddend. .Iadd.34.The data storage method set forth in claim 33 further comprising initialtrack centerline calibrating steps of:calculating and recording headposition correction signals for a plurality of tracks lying in anoutwardly lying band of tracks, calculating and recording head positioncorrection signals for a plurality of tracks lying in an inwardly lyingband of tracks, averaging said correction signals of said outwardtracks, and averaging said correction signals of said inward tracks, andspreading the difference between said two averages uniformly over thetracks lying between said two bands..Iaddend. .Iadd.35. A fine positionservo system for promoting and maintaining transducer-to-trackcenterline alignment in a data storage device, said data storage deviceincluding at least one data storage disk with a major surface having amultiplicity of data tracks, motor means for rotating the data storagedisk at a predetermined proper angular velocity, index marker means forgenerating an index signal once per revolution of the data storage disk,a transducer for reading data from and writing data to the multiplicityof data tracks, a mover means for moving the transducer to a selectedone of the multiplicity of data tracks and for holding the transducerwithin the boundaries of the selected data track, and an interfacestructure through which the data storage device can communicate with ahost system, said fine position servo system comprising:a single servosector disposed on the major surface of the data storage disk at alocation which is determined in time by the index signal, said singleservo sector having prerecorded therein track centerline information forthe selected data track readable by the transducer; and servo controlmeans which receives the index signal for responsively marking in timesaid location of said single servo sector and for indicating the end ofthe servo sector, after which data can be written to or read from theselected data track using a substantially unrestricted data format, saidservo control means including a circuit connecting structure over whichthe index signal is sent to said servo control means rather than to theinterface structure, said servo control means also including a servocircuit means connected to the transducer and to the mover means forenabling the mover means to move the transducer into alignment with thecenterline of the selected data track in response to said trackcenterline information read by the transducer..Iaddend. .Iadd. . A fineposition servo system as set forth in claim 35, wherein said mover meansfor moving the transducer to a selected one of the multiplicity of datatracks includes an electromechanical mover under the control of a coarseposition servo system and a driver means for actuating the servoelectromechanical mover, said servo circuit means in said servo controlmeans including correction means connected to said driver means foractuating the electromechanical mover to move the transducer intoalignment with the centerline of the selected data track..Iaddend..Iadd.37. A fine position servo system as set forth in claim 35, whereinthe selected data track to which the mover means moves the transducermay comprise any one of the multiplicity of data tracks and said singleservo sector has prerecorded therein track centerline information forall of the multiplicity of data tracks..Iaddend. .Iadd.38. A fineposition servo system for promoting and maintaining transducer-to-trackcenterline alignment in a data storage device, said data storage deviceincluding at least one data storage disk with a major surface having amultiplicity of data tracks, motor means for rotating the data storagedisk at a predetermined proper angular velocity, a transducer forreading data from and writing data to the multiplicity of data tracks, amover means for moving the transducer to a selected one of themultiplicity of data tracks and for holding the transducer within theboundaries of the selected data track, and an interface structurethrough which the data storage device can communicate with a hostsystem, said fine position servo system comprising:a single data maskedservo sector disposed on the major surface of the data storage disk andhaving prerecorded therein track centerline information for the selecteddata track readable by the transducer; and servo control means formarking in time the location of said single data masked servo sectorafter which data can be written to or read from the selected data trackusing a substantially unrestricted data fromat, said servo control meansincluding a servo circuit means connected to the transducer and to themover means for enabling the mover means to move the transducer intoalignment with the centerline of the selected data track in response tosaid track centerline information read by the transducer..Iaddend..Iadd.39. A device for storing data, said device comprising:a frame; atleast one data storage disk mounted for rotation about a spindlesupported on said frame, said data storage disk having at least onesurface which contains a multiplicity of concentric data tracks, meansfor rotating said data storage disk about said spindle at apredetermined proper angular velocity, an index marker means forgenerating an index pulse once during each revolution of said datastorage disk about said spindle, at least one read/write head means forreading from or writing data to said data tracks on said data storagedisk, an interface structure through which the device can communicatewith a host system, a head carriage assembly including a supportstructure on which said read/write head means is mounted, anelectromechanical mover means for moving said head carriage assemblyradially over said surface of said data storage disk such that saidread/write head means can be shifted from a departure track to aselected destination track, a programmed microprocessor means forgoverning the operation of said electromechanical mover means, anoptical encoder means for detecting movement of said electromechanicalmover means and for generating quadrature signals in a response thereto,which quadrature signals provide information relating to the position ofthe read/write head means over said surface of said data storage disk, acoarse position servo circuit means connected to receive said quadraturesignals generated by said optical encoder means and to generate ananalog servo waveform in response thereto,a summing circuit meansconnected to said electromechanical mover means and said coarse positionservo circuit means for receiving said analog servo waveform and forstabilizing said read/write head means within the boundaries of saidselected destination track in response to said analog servo waveform anda fine position servo circuit means for promoting and maintainingalignment between said read/write head means and the centerline of saidselected destination track, said fine position servo circuit meansincluding: a single servo sector containing track centerline informationin the form of servo bursts prerecorded on said surface of said datastorage disk at a location which immediately follows in the time theoccurance of said index pulse such that said track centerlineinformation can be read by said read/write head means as said read/writehead means passes over said single servo sector during each revolutionof said data storage disk, servo control means which receives the indexsignal for responsively marking in time said location of said singleservo sector and for indicating the end of the servo sector, after whichdata can be written to or read from said selected data track using asubstantially unrestricted data format, said servo control means havinga circuit connecting structure over which said index pulse is sent tosaid servo control means rather than to said interface structure, saidservo control means also having a servo circuit means for enabling saidelectromechanical mover means to move said read/write head means intoalignment with the centerline of said selected data track, said servocircuit means including said microprocessor means, a sample and holdmeans connected to said read/write head means for holding said trackcenterline information read by said read/write head means as saidread/write head means passes over said single servo sector and forsupplying said microprocessor means with a comparison signal having avalue dependent upon the position of said read/write head means relativeto said centerline of said destination track, and digital-to-analogconverter means connected to said microprocessor means for supplying anoffset value to said summing circuit means in response to saidcomparison signal such that said electromechanical mover means maintainssaid read/write head in alignment with said centerline of said selecteddestination track..Iaddend. .Iadd. In a data storage device including arotating rigid magnetic medium data storage disk, a read and write headheld in close proximity to the surface of said disk by the air-bearingeffect, and a head carriage mechanism for positioning said head at oneof a multiplicity of concentric data tracks during data read and/orwrite operations in substantially unrestricted data format within eachtrack and for moving said head from track to track during track seekingoperations of said device, the improvement comprising:driver meansdrivingly connected to said head carriage mechanism for arcuately movingsaid head from one track to a selected other track located beyond apredetermined minimum radial distance during an opened loop servo trackseek movement characterized by a first spatial increment of maximumforward direction acceleration of said carriage for about half theradial distance between said tracks and by a second similar increment ofmaximum reverse direction acceleration until said mechanism reaches thevicinity of said selected other track, and for closed loop generallyconstant velocity microstepping of said mechanism within said minimumradial distance in small radial increments from track to track untilsaid other track is reached, and for maintaining said head accuratelywithin a selected track during data read and write operations, a summingcircuit connected to drive said driver means, coarse servo control loopmeans including coarse position sensor means mechanically linked to saidcarriage mechanism for sensing radial position of said head relative tosaid tracks, and coarse position feedback control loop means connectedto said driver means through said summing circuit for controlling saiddriver means to keep said head positioned within each said selected oneof said defined tracks during read and write operations, index detectormeans electromechanically coupled to said disk for detecting eachrotation thereof and generating an index signal, fine position servocontrol loop means including a single data-masked servo sector region onthe surface of said disk containing servo information therein, sampleand hold means connected to said head for sampling and holding saidservo information read in said servo sector for each revolution of saiddisk, correction signal generator means connected to said sample andhold means for generating offset signals, and analog to digitalconverter means connected to said correction signal generator means, forconverting said offset signals into digital values, programmed digitalmicroprocessor means connected to said analog to digital convertermeans, said index detector means, said coarse servo control loop meansand to a source of an externally supplied digital track selectioncommand signal, and including clocked latch means, and digital toanmalog converter means drivingly connected to said driver means throughsaid summing network for calculating and supplying a digital headpositioning signal to said driver means in order to move said head froma predetermined departure track to a predetermined destination track,and for calculating and applying a fine position offset signal to saiddriver means through said digital to analog converter means and saidsumming network to maintain said head in centerline alignment in saidtrack during read and/or write operations interactively with said coarseservo control loop means..Iaddend.